A cutting element typically has cylindrical cemented carbide substrate body having an end face (also referred to herein as an “interface surface”). An ultra hard material layer, such as polycrystalline diamond or polycrystalline cubic boron nitride, is bonded on the interface surface forming a cutting layer. The cutting layer can have a flat or a curved interface surface.
Generally speaking the process for making a cutting element employs a body or substrate of cemented tungsten carbide where the tungsten carbide particles are cemented together with cobalt. The carbide body is placed adjacent to a layer of ultra hard material particles such as diamond of cubic boron nitride (CBN) particles and the combination is subjected to a high temperature at a high pressure where diamond or CBN is thermodynamically stable. This results in recrystallization and formation of a polycrystalline diamond or polycrystalline cubic boron nitride layer on the surface of the cemented tungsten carbide. This ultra hard material layer may include tungsten carbide particles and/or small amounts of cobalt. Cobalt promotes the formation of polycrystalline diamond or polycrystalline cubic boron nitride and if not present in the layer of diamond or CBN, cobalt will infiltrate from the cemented tungsten carbide substrate.
The cemented tungsten carbide substrate is typically formed by placing tungsten carbide powder and a binder in a mold and then heating to the binder melting temperature causing the binder to melt and infiltrate the tungsten carbide particles fusing them together and cementing the substrate. Alternatively, the tungsten carbide powder may be cemented by the binder during the high temperature, high pressure process used to re-crystalize the ultra hard material layer. In such case, the substrate material powder along with a binder are placed in a can typically formed from a refractory metal, forming an assembly. Ultra hard material particles are provided over the substrate material to form the ultra hard material polycrystalline layer. The entire assembly can is then subjected to a high temperature, high pressure process forming a cutting element having a substrate and a polycrystalline ultra hard material layer over it.
The problem with many cutting elements is the development of cracking, spalling, chipping and partial fracturing of the ultra hard material cutting layer at the layer's region subjected to the highest impact loads during drilling, especially during aggressive drilling. To overcome these problems, cutting elements have been formed having a non-planar substrate interface surface having grooves or depressions. Applicant has discovered that these grooves or depressions cause the build-up of high residual stresses on the interface surface leading to premature interfacial delamination of the ultra hard material layer from the substrate. Delamination failures become more prominent as the thickness of the ultra hard material layer increases. However, it is believed that the impact strength of the ultra hard material layer increases with an increase in the ultra hard material layer thickness.
Another problem with an increase in the thickness of the ultra hard material layer, is that the edges of the ultra hard material furthest from the substrate are starved of cobalt from the substrate during the sintering process resulting in the ultra hard material edges having decreased strength. Consequently, the edges become brittle and have lower impact strength and wear resistance. In an effort to solve this problem, some cutting elements incorporate a frustum-conical section defined on the substrate interface surface that is surrounded by the ultra hard material layer. In this regard, the edges of the ultra hard material layer are closer to the cobalt source, i.e., the frustum conical section of the substrate. However these cutting elements are also subject to the build-up of high residual stresses on the interface region leading to premature interfacial delamination of the ultra hard material layer.
Consequently, a cutting element is desired that can be used for aggressive drilling and which is not subject to early or premature failure, as for example by delamination of the ultra hard material layer from the substrate, and which has sufficient impact strength resulting in an increased operating life.